In superconductors, there is a critical temperature or Tc at which resistance to the passage of electricity disappears. Conventional superconducting metals, alloys, and compounds have critical temperatures ranging from just above absolute zero to about 15-20.degree. K. Practical applications for such superconductors are limited because they are operative only at extremely low temperatures.
A new class of superconductors, designated high-Tc superconductors, has recently been discovered and is being extensively investigated. The members of this class have much higher critical temperatures making it possible to use them in devices of commerce. Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7, for example, has a critical temperature of about 93.degree. K.
In most of their commercial applications, superconductors are or will be electrically connected to other components of the device containing them. It is essential that the resistivity of the connections be low, particularly where high currents are to be carried, such as in transmission lines, generators and motors, and in energy storage devices and other magnetic applications. Low resistivity connections are required for superconductors which are part of an integrated circuit in high density, high speed computers to reduce the heat loads in such computers.
Low resistivity contacts ar especially important for high-Tc superconductors, since even moderate resistance-caused heating can raise the temperature of a superconductor enough to significantly lower its critical-current density. Low resistivity contacts are required for high-Tc superconductors in both bulk applications, such as electromagnets, and in thin-film devices, such as computers. Contact resistivity is expressed in terms of surface resistivity .rho..quadrature. = RA, where R is the contact resistance, and A is the contact area. For small magnet applications at liquid nitrogen temperatures, contact resistivities less than about 10.sup.-5 .OMEGA.-cm.sup.2 are required to limit heating at the contact to acceptable levels. For circuit board applications, contact resistivities less than about 10.sup.-4 .OMEGA.-cm.sup.2 are required for the external wire-bond connections to superconducting integrated-circuit chips.
Contacts made with indium solder, silver paint, direct wire bonds and pressure contacts have a contact surface resistivity typically in the range 10.sup.-2 to 10 .OMEGA.-cm.sup.2, several orders of magnitude too high for practical applications.
It is known to deposit metals on ceramic components to provide a situs for electrical connections to leads fabricated of copper or other conductive metal. Deposition by sputtering is particularly desirable because the metal deposited strongly adheres to the ceramic substrate. It is known from U.S. Pat. No. 4,337,133 to use sputtered gold as the metal to prepare conductive electrical contact surfaces. It is known from the paper titled "Metallization Of Ceramics For Electronic By Magnetron-Plasmatron Coating" by Schiller et al in Thin Films, 72, 313-326 (1980), that ceramics having silver deposited thereon exhibit good solderability. Various other methods of joining or soldering metals to refractory materials are disclosed in U.S. Pat. No. 3,915,369 and 3,993,411.
Typical leads for connecting components in an electrical device or system are copper wires, silver wires, aluminum wires, gold-plated wires and the like. However, connecting such leads to high-Tc superconductors by conventional means such as soldering using flux-containing solders of the type described in U.S. Pat. No. 3,703,254, or even with indium-based solders, results in a relatively high resistivity connection which can adversely affect desirable properties of the superconductor. High resistivity connections can result even if the superconductor contains a metal contact pad.